1. Field of the Invention
The present invention relates to a wire boding method for connecting two electrodes together with a wire, a surface acoustic wave apparatus and a method for producing a surface acoustic wave apparatus.
2. Description of the Related Art
A wire bonding method has been widely used for electrically connecting electrodes that are separated from each other. Since the surface of a bonding electrode is covered with an oxidized film, in order to attach a bonding wire thereon, the oxidized film must be removed. In an ultrasonic wire bonding method, ultrasonic oscillations are applied to a boding wire so that an oxidized film on the surface thereof is destroyed by sliding of the boding wire, thereby revealing an activated metallic surface. Then, a temperature increases due to frictional heat and a plastic flow causes localized melting, diffusion, and so forth, so that the junction is achieved by metallic bonding.
When such a wire bonding method is applied to surface acoustic wave elements which include an electrode made of a material such as tantalum having a larger specific weight than that of a piezoelectric substrate to excite a Shear Horizontal (xe2x80x9cSH-typexe2x80x9d) surface acoustic wave (Love wave, etc.), as disclosed, for example in U.S. Pat. No. 5,953,433, and U.S. Pat. No. 5,847,486, there arises a problem that a bonding wire is not connected firmly to an electrode of the surface acoustic wave element.
In order to overcome the problems described above, preferred embodiments of the present invention provide a wire bonding method by which a bonding wire is strongly connected to a tantalum electrode of a surface acoustic wave element, and provide a surface acoustic wave apparatus formed by using the above-described wire bonding method to have high junction strength and high electrical reliability and a method for producing a surface acoustic wave apparatus.
According to one preferred embodiment of the present invention, a wire bonding method for ultrasonically joining two connecting electrodes with a wire includes the steps of forming at least one of the connecting electrodes with tantalum and forming the wire of one of a metallic wire having a fracture load of about 21 g or greater and a metallic wire having a fracture stress of 290 N/mm2 or greater.
According to preferred embodiments of the present invention, by using a wire having large strength (breaking stress is large), plastic deformation is minimized and ultrasonic energy is effectively applied to the junction surface at the tantalum layer. Thereby, the strong junction due to the metallic bonding between the bonding wire and the tantalum layer is achieved, so that the bonding wire can be directly and reliably joined to the tantalum layer.
Furthermore, it is not necessary to form an additional electrode made from a material such as Au or Al on the tantalum layer, so that the number of manufacturing steps is greatly reduced, resulting in reduced cost.
For the fracture load, it is preferable that the bonding wire use an aluminum alloy wire, in particular use an Alxe2x80x94Si wire. By adding a very small amount of Si to the aluminum, the fracture load is greatly improved.
The wire bonding method according to preferred embodiments of the present invention may be preferably applied to a surface acoustic wave apparatus. That is, the method may be applied to a surface acoustic wave apparatus including a package having an element-mounting surface and a surface acoustic wave element fixed on the element-mounting surface of the package, the surface acoustic wave element including a piezoelectric substrate and IDTs and bonding electrodes disposed on the upper surface of the piezoelectric substrate, the bonding electrode of the surface acoustic wave element and an external connecting electrode of the package are connected together by an ultrasonic bonding wire method according to other preferred embodiments of the present invention. Thus, such a surface acoustic wave apparatus is prepared by a process including the steps of using tantalum for the IDT and bonding electrodes of the surface acoustic wave element, and using one of a bonding wire having a fracture load of about 21 g or greater and a bonding wire having a fracture stress of about 290 N/mm2 or greater for a bonding wire. Thereby, the large junction strength between the bonding wire and the bonding electrode formed by a tantalum layer can be obtained, so that even when a dropping impact is applied thereto, the bonding wire can be prevented from being removed from the bonding electrode.
For the purpose of illustrating the present invention, there is shown in the drawings several forms that are presently preferred, it being understood, however, that the present invention is not limited to the precise arrangements and instrumentalities shown.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the detailed description of preferred embodiments below with reference to the attached drawings.